How can I create slope with precise angle in edit mode?
If I understand correctly the goal is to kind of rotate a face but without connected faces distorting:



One way to do that is to use Shear tool; select the desired face, enter desired axis and amount of effect the face to be "rotated". 

Note that as it works regarding current view, you should position viewport point of view accordingly, most likely in preset orthographic view. If shearing the top face of a cube by Y axis enter orthographic Front view and while shearing enter Y axis (not seen on gif here) and then amount of effect.



Another much simpler way is not rotate enything but just grab edges by Z axis - one up, another down. It won't be rather simple to guess how much though as this will be moving edges, not rotating face.